Antidepressants currently available are very similar toward their mechanism of action and are more or less effective. One major problem is their long latency to provide a therapeutic effect due to adaptations of pre and post synaptic locations. In an animal model, we recently discovered that the agonist RS67333 of the 5-HT4 receptors was able to produce in three days the same antidepressant effects that normally take two to three weeks to appear with the currently available antidepressants. In addition, we found that the antidepressant effects of this agonist had a resistance to tolerance. There are others agonists of the same receptor such as prucalopride, which does not produce antidepressant effects as RS67333. Since the effectiveness of prucalopride to stimulate 5-HT4Rs is similar if not greater than RS67333, we stated the hypothesis that the 5-HT4 receptor could adopt different active conformations following its activation by various agonists. We decided to explore the major functional responses of 5-HT4B by observing their regulatory and signaling properties.
We showed that the B isoform of the 5-HT4, being highly expressed in the limbic system, has a different signaling and regulation depending on the ligand. Our results indicate that each of the agonists tested (5-HT, RS67333, ML10302, Zacopride, Prucalopride) distinctively modulate cAMP production and receptor internalization. The results have clearly identified that agonists differed in potency and efficacy. Moreover, the order of effectiveness of agonists to modulate the cAMP pathway was (prucalopride> zacopride = 5-HT = ML10302> RS67333) different from their order of effectiveness in inducing receptor regulation by internalization (5-HT> Zacopride> Prucalopride> RS67333 = ML10302). Thus, we have shown that 5-HT4Rs adopt conformations that are ligand-specific. This implies that functional selectivity is an important factor in the mechanisms of antidepressant action of this receptor agonists.